1. Field
The present invention relates generally to communication systems, and more particularly to techniques for estimating the velocity of a terminal in a wireless communication system.
2. Background
A common means by which to locate a terminal is to determine the amount of time required for signals transmitted from multiple sources at known locations to reach the terminal. One system that provides signals from a plurality of transmitters of known locations is the well-known Global Positioning System (GPS). Satellites in the GPS system are placed in precise orbits according to a GPS master plan. The locations of the GPS satellites can be identified by different sets of information (referred to as the Almanac and Ephemeris) transmitted by the satellites themselves. Another system that provides signals from transmitters (i.e., base stations) at known earth-bound locations is a wireless (cellular) communication system.
Signals from satellites and/or base stations may be used to estimate the location of a terminal. By receiving and processing the signals transmitted from these transmitters, the amount of time required for the signals to travel from the transmitters to the terminal may be estimated and used to compute the distances (or ranges) between the transmitters and the terminal. The signals themselves may further include information indicative of the locations of the transmitters. By accurately determining the distances to three or more transmitters at known locations, the position of the terminal may be determined using trilateration.
In certain instances and for certain applications, the velocity of the terminal may also need to be ascertained. In one simple technique for estimating velocity, a series of position fixes are determined for the terminal and used to estimate its velocity. However, this technique has several shortcomings. One shortcoming relates to the use of position fixes to estimate velocity. If the position fixes are determined at short time intervals, then small errors in the position fixes may result in large errors in the velocity estimate. However, if the position fixes are determined at longer time intervals, then the position fixes may be more indicative of the average velocity of the terminal instead of the instantaneous velocity.
A second shortcoming is related to the shared resources at the terminal. In many terminal designs, some or all of the elements used for voice and/or data communication are also used for position determination These terminal designs typically do not allow the shared elements to be used simultaneously for both communication and position determination. Consequently, communication is typically inhibited while position is being determined, and vice versa Obtaining several consecutive position fixes would then require the terminal to stay in a GPS mode for an extended period of time or to repeatedly interrupt communication.
There is therefore a need in the art for techniques to efficiently and accurately estimate the velocity of a terminal in a wireless communication system.
Aspects of the invention provide techniques to estimate the velocity of a terminal in a wireless communication system. Movement by the terminal results in a Doppler shift in the frequency of each transmitted signal received at the terminal. This Doppler frequency shift is related to the terminal""s velocity, which may be accurately estimated by processing the received signal to provide a set of frequency errors in the transmitted signals (as received at the terminal) for a number of satellites. Various scenarios are described in further detail below, and the terminal""s velocity may be estimated (1) based on signals from both base station and satellites or based only on signals from satellites and (2) for a 3-dimensional (e.g., earth-centered, earth-fixed) or a 2-dimensional (e.g., east, north) frame.
A specific embodiment of the invention provides a method for estimating the velocity of a terminal in a wireless communication system. In accordance with the method, the positions of the terminal a base station, and each of two or more satellites are initially determined. A residual rate of change of pseudo-range may also be determined for each satellite. A set of equations is then formed based on the determined positions of the terminal, the base station, and the satellites and the determined residual rates of change of pseudo-ranges for the satellites. The velocity of the terminal may thereafter be estimated based on the set of equations.
To determine the residual rates of change of pseudo-ranges for the satellites, the received signal (which includes the signals transmitted from the satellites) is initially downconverted to provide a baseband signal. The frequency error of the baseband signal is then determined for each satellite. The Doppler shift in the frequency of the signal from each satellite is also estimated. The residual rate of change of pseudo-range for each satellite is then determined based on the estimated baseband frequency error and Doppler frequency shift for the satellite.
For certain scenarios, the terminal""s velocity may be estimated without using the base station. In this case, the residual rates of change of pseudo-ranges are determined for three or more satellites, and the frequency error in the oscillator used to downconvert the received signal becomes an additional unknown that can be solved for using an additional satellite measurement. The velocity estimation techniques are described in further detail below.
The invention further provides other methods, computer program products, receiver units, terminals, and apparatus and elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.